One technique of encapsulating biologically active materials for sustained-release purposed is disclosed in U.S. Pat. Nos. 4,016,100; 4,145,410; 4,235,871; and 4,241,046. In these applications polymer-water preparations or systems are utilized as encapsulating materials. These preparations are, however, thermodynamically unstable (dispersions, emulsions and vesicles) and consist of at least two phases.
The present invention is based on the use of a fundamentally different system, viz. a thermodynamically stable one-phase composition having a well-defined structure, by which it has turned out possible to eliminate or at least drastically reduce disadvantages associated with the above-mentioned prior art compositions.
The new composition or system used according to the present invention is a non-toxic liquid phase formed from certain amphiphilic substances and a polar liquid and is called an L2-phase. The L2-phase is a phase known per se but as far as we know of it has previously not been used for the purposes of the present invention. However, for a better understanding of the invention the present information can be given concerning amphiphilic substances and the L2-phase.
Amphiphilic substances are substances with hydrophilic as well as hydrophobic (lipophilic) groups and such substances spontaneously tend to self-associate in aqueous systems forming various types of aggregates. The L2-phase is one such phase. The L2-phase is a liquid phase with water-aggregates in a hydrocarbon-continuous medium (see Ekwall, P., Advances in Liquid Crystals, Ed. G. W. Brown, Academic Press, New York, 1975). The phase can coexist in equilibrium with water or a dilute aqueous solution. It is known that edible oils, like soybean oil, and water can form such a phase in the presence of monoglycerides of unsaturated fatty acids, such as sunflower oil monoglycerides (see Fontell et al. J. Colloid Interface Sci. 93 (1983) 453). Further information about L2-phases will be given below in connection with the disclosure of the invention.